In prior art techniques, as disclosed in Japanese laid-open Patent Gazette Nos. 246087/1996 and 19757/1997, as cylinder liners of reciprocating internal combustion engines, cylinder liners made of hyper-eutectic aluminum-silicon alloy by an aluminum alloy casting method (hereinafter, called A1 cylinders) have been developed. Such liners are excellent in anti-abrasion property, have less consumption of lubricant and less abrasion on the piston, and promote light engine weight, and thus are increasingly being used.
A piston for such an A1 cylinder is normally provided with a set of three piston rings. The three piston rings include a top ring (pressure ring) with a surface which is nitriding-treated, a second pressure ring of which the outer peripheral sliding surface is chromium-plated, and an oil ring forming a third ring (oil scraping ring). Incidentally, as opposed to an A1 cylinder such as, for example, one made of A390 alloy, a pressure ring made of acicular cast iron without plating thereon could be employed as the second pressure ring.
The second pressure ring needs to be airtight for backing up the top ring and to provide a degree of scraping for scraping a little lubricant that the oil ring may possibly leave unscraped.
As for the second pressure ring, therefore, the outer periphery thereof should slant so as to be downwardly divergent or slant with an undercut, and the lowermost portion of its sliding surface should be formed with a sharp edged shape. On the other hand, in order to prevent the cylinder from being scratched, the sliding surface should be chromium-plated and the slant shaped surface should be polished to make the edged portion sharply and smoothly finished. Therefore, it has been hard in practice to satisfy requirements for both providing the scraping property and preventing longitudinal scratching of the bore (cylinder side).